MODEL DIAGNOSTIC DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1226 AM EDT TUE SEP 01 2015 VALID SEP 01/0000 UTC THRU SEP 04/1200 UTC ...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR INGEST... 00Z NAM/GFS EVALUATION AND PRELIMINARY PREFERENCES UPPER TROUGH FORMING ACROSS THE PACIFIC NORTHWEST... PREFERENCE: GFS OR ECMWF CONFIDENCE: ABOVE AVERAGE THE GFS AND 12Z ECMWF ALIGN WELL WITH THE MODEL CONSENSUS INCLUDING THE ENSEMBLE MEANS WITH NEAR TO BELOW NORMAL SPREAD BY DAY 3...WITH THE NAM TRENDING FASTER BUT STILL LAGGING THE CONSENSUS... WHICH COULD BE A RESULT ITS OCCASIONAL SLOW BIAS. THUS...RECOMMEND EITHER THE GFS OR ECMWF. UPPER LOW OVER THE LOWER MS VALLEY... PREFERENCE: HALFWAY BETWEEN THE GFS AND 12Z ECMWF CONFIDENCE: BELOW AVERAGE THE SPREAD REMAINS VERY HIGH CONCERNING THE DETAILS OF THE STRUCTURE OF THE EVOLVING LOW. HOWEVER...THERE ARE SOME INDICATIONS TO LEAN SOMEWHERE BETWEEN THE GFS AND 12Z ECMWF....WITH THE NAM TRENDING EAST AND AWAY FROM ITS MOST WESTWARD POSITION NEAR THE TEXAS COAST BY DAY 3...THE GFS TRENDING EASTWARD...AND THE 18Z PARALLEL GEFS MEAN SUPPORTING AN INTERMEDIATE SOLUTION BETWEEN THE 18Z OPERATIONAL GEFS MEAN AND THE 12Z ECMWF ENSEMBLE MEAN. THUS...A SOLUTION ABOUT MIDWAY BETWEEN THE GFS AND 12Z ECMWF IS PREFERRED. REMNANTS OF ERIKA... PREFERENCE: GFS CONFIDENCE: AVERAGE THE GFS ALIGNS BEST WITH THE REMNANTS OF ERIKA...WITH THE NAM A CLOSE SECOND AND THE 12Z ECMWF ALREADY TOO FAR NORTHEAST. THUS...THE ECMWF CAN BE RULED OUT. BETWEEN THE GFS AND NAM...THE GFS APPEARS TO BETTER CAPTURE THE SUBSIDENCE RING DEVELOPING OVER SOUTHERN FLORIDA PARTICULARLY THROUGH TODAY AND TONIGHT. THUS...RECOMMEND THE GFS. SHORTWAVE PATTERN ACROSS THE GREAT LAKES/OH VALLEY THU-FRI... PREFERENCE: ENSEMBLE MEANS CONFIDENCE: BELOW AVERAGE THE GFS AND 12Z ECMWF REPRESENT THE OUTER EDGES OF THE GUIDANCE...WITH THE GFS POSSIBLY TOO QUICK TO PROGRESS A SHORTWAVE TROUGH INTO A LONGWAVE RIDGE OVER CENTRAL CANADA BY DAY 3...THUS BUILDING THE SHORTWAVE RIDGE ACROSS THE GREAT LAKES...WHILE THE 12Z ECMWF IS NEAR THE STRONG EDGE OF THE GUIDANCE WITH A SHORTWAVE TROUGH EXITING THE ROCKIES AND REACHING THE CENTRAL PLAINS. THE NAM IN BETWEEN THESE MORE EXTREME SOLUTIONS BUT HAS ITS OWN ISSUES WITH A SUSPICIOUSLY STRONG AND NORTHERN LEE LOW DEVELOPING OVER THE NORTHERN PLAINS...THUS POSSIBLY IMPACTING TEMPS/WINDS OVER THE UPPER MIDWEST. THUS...THE NAM IS NOT RECOMMENDED EITHER. TO ADDRESS THESE CONCERNS... RECOMMEND THE ENSEMBLE MEANS UNTIL BETTER DETERMINISTIC AGREEMENT DEVELOPS. www.wpc.ncep.noaa.gov/html/model2.shtml... www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml... JAMES